Plotting of streamz apply is odd

[martindurant]

In the following code

data = a streamz dataframe with time-based index and example
recent = data.window(value='5m')
ts = recent.apply(lambda d: d.time.resample('10s').count())

• ts.hvplot(kind='table') works (but time index doesn't show, #199 )
• ts.hvplot(kind='bar') fails with

  
  ~/anaconda/envs/py36/lib/python3.6/site-packages/pandas/core/internals/managers.py in insert(self, loc, item, value, allow_duplicates)
  1147         if not allow_duplicates and item in self.items:
  1148             # Should this be a different kind of error??
  -> 1149             raise ValueError('cannot insert {}, already exists'.format(item))
  1150 
  1151         if not isinstance(loc, int):

ValueError: cannot insert time, already exists

- `ts.hvplot()` does not error but does not work

bokeh backend could not plot any Elements in the Overlay. :DynamicMap [] :NdOverlay [Variable]

[philippjfr]

Most of these work for me now, I'll make a quick PR for the table issue.